The present invention relates to a method of heating a structure, e.g. to undergo a subsequent hot forming process, and to an apparatus for carrying out the method.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The mechanical resistibility of steel structures can be enhanced by hardening the material through heating and subsequent rapid quenching. The change in position of the carbon atoms in the metal lattice is the reason for the increase in hardness and begins when the austenitic temperature has been reached. The following quenching leads to a martensitic microstructure that significantly increases the strength of the structure. In the case of thin-walled steel structures, for example steel sheet blanks, the application of compression molding or press hardening has been shown reliable to hot-form metal sheets. After being heated, the blank is placed in a shaping tool to undergo a subsequent forming and hardening through quenching. In particular the automobile industry increasingly demands for ecological and economical reasons that the thus manufactured high-strength body structures exhibit a beneficial ratio of strength to weight.
The use of elongated or round continuous furnaces, such as for example rotary hearth furnaces or roller hearth furnaces, placed upstream of the shaping and hardening processes has been proposed to continuously supply the press tools with heated steel sheets. A steel sheet placed in the continuous furnace is moved through the furnace by a transport device and heated thereby under the furnace atmosphere and maintained at this temperature. The structure reaches its desired temperature for austenitization prior to its removal via the furnace exit.
Continuous furnaces are in general bulky and require much space. The transport systems typically used in such furnaces are subject to increased wear regardless of their position because they operate in a continuous mode and are continuously exposed, at least in part, to the hot furnace atmosphere. Due to its dimensions, the overall facility is considered static and inflexible and complicated so as to render the facility difficult to modify and reposition. As a result, the facility is not only cost-intensive but has a large footprint and is difficult to integrate in existing constructions. Maintenance works require a cool down of a relatively bulky heated mass which subsequently has to be completely heated again. The result is excessive energy consumption. Also the passage time of the structures to be heated inside the furnace atmosphere is long so that the structures show a tendency for scale formation and surface decarburization.
It would therefore be desirable and advantageous to address prior art shortcomings.